well there's the text editor, you could split your view3d and open a new text block next to it. Then type your notes in there. Of course, this isn't as convenient as "floating" notes in 3d view directly.

If you're fine with handwriting, you can use the grease pencil tool.

I looked a bit into grease pencil API, and think it would be possible to create an addon that would convert text from an input field into grese pencil strokes - nicer than handwriting i guess

Someone would have to create line data for every letter though, probably ascii only, but still a lot of work. Would do this as a paid job if you want.